Digital encoders or resolvers generate a digital output signal that indicates the position of an object, such as the linear position of a slide or the angular rotation of a shaft. The digital output signal is usually generated by a series of tracks, one track for each bit of the signal. The bit pattern on the tracks can be encoded by conducting/nonconducting elements. For example, a 1 state may be represented by a conducting element, and a 0 state by a nonconducting element. The digital code may then be read by an array of electrical wipers with the conducting elements having a common electrical return.
More recently, optical tracks have been used for encoders, wherein a 1 state is represented for example by a transparent element of the track, and a 0 state by an opaque element of the track. Alternately, reflecting and nonreflecting elements can be used to represent the data. The tracks may be illuminated by individual light sources, e.g., LEDs or incandescent bulbs, or by a common light source. Optical transmission or reflection may be read by a common detector, or by an array having one detector for each track. The detector outputs are converted into a 1 or 0 digital level by suitable electronics.
Most prior optical encoder systems have been interrogated or read via an electrical interface. Electrical interconnections are vulnerable to EMI and EMP, and, in some instances, electrical power may not be available at the location of the encoder. Therefore, for many applications, an electrically passive encoder, i.e., an encoder that requires neither electrical interconnects for interrogation, nor electrical power for operation, would be desirable. One method of obtaining the goal of an all optical encoder is to use optical fibers to interface the sensor with the optical sources and detectors. A number of such optical encoding systems have been proposed, including systems based upon optical time domain reflectometry, and systems based upon wavelength division mulitplexing (WDM).
In prior WDM systems of this type, a plane diffraction grating has been used as the dispersion element to demultiplex the incoming light into separate components based on wavelength, and to multiplex the light reflected or transmitted by the encoder tracks.
A fundamental problem common to such prior techniques is that optical elements are required between the optical fibers and the diffraction grating, and between the diffraction grating and the encoder tracks. In a system where the encoder tracks alternately transmit and block the light, four such optical elements are required. The first element collimates the light emerging from the first optical fiber before the light is incident on the diffraction grating. The second element focuses the component wavelengths of the diffracted light onto the associated encoder tracks. The third element collimates the light at the component wavelengths which are transmitted by the encoder tracks prior to the light being multiplexed by the diffraction grating. The fourth element focuses the collimated multiplexed beam after diffraction by the diffraction grating onto the second optical fiber. In a system where the encoder tracks either reflect or absorb light at the component wavelengths, light at the component wavelengths retraces the same path after reflection, and the number of optical elements is reduced to two. In both the transmission and reflection optical systems, the necessity for collimating and focusing optical elements increases both the optical complexity and size of the read head.
The reflection optical system is desirable as the number of optical components is half the number required in the transmission optical system. In such prior WDM systems, a single optical fiber has served as both the first and second optical fibers. An undesirable feature of such an arrangement is that at some point in the sensor system, light from the radiation source has to be coupled into the common optical fiber, and the reflected light from the sensor has to be coupled out of the common optical fiber to the demultiplexer/detector. This invariably results in an optical power system loss of at least -6 dB.